1. Field of the Invention
The present invention relates to an acetabular socket (shell) for an artificial hip joint.
2. Prior Art
As shown in FIG. 1, an artificial hip joint comprises artificial members used to replace and restore a hip joint located between an acetabular bone H and a femur F. An acetabular socket A is fitted and seated in a concave socket receiving section R provided in the acetabular bone H. A stem head SH provided at the upper end of a stem S embedded in a canal of the femur F is rotatably fitted in the above-mentioned acetabular socket A via a seat C made of high-density polyethylene. The acetabular socket A, stem head SH, and stem S are made of materials non-harmless to a living body, either of metal such as titanium, titanium alloy, or cobalt-chromium alloy, or of ceramics such as alumina ceramics or zirconia ceramics. The acetabular socket A is fitted in and secured to the concave socket receiving section R of the femur F to rotatably support the stem head SH. The socket is positioned at the interface between the living body and the artificial member and plays a very important role in receiving the dynamic load applied to the femur F. The conventional-type acetabular socket A takes a form of an inverted bowl-shaped shell or a hemispherical shell, is fitted in the concave socket receiving section R, and secured to the acetabular bone H by using screws B as shown in FIG. 6. This conventional type has the following problems:
(i) Referring to FIG. 7, a cortex bone H1 located on the normal acetabular bone H is a hard layer and a cancellous bone H2 inside the cortex bone H1 is a soft layer. The former can support external force, but the latter cannot. In the case of the conventional type, since the opening edge a1 (see FIG. 2) of the acetabular socket A is on the hemispherical surface of the socket A, the main area of the socket A contacts the cancellous bone H2 under pressure as shown in FIG. 6. In addition, since the opening edge a1 does not contact the cortex bone H11 which functions as an opening edge a1 of the concave socket receiving section R, the area of the cortex bone H1 for sturdily supporting the socket A is insufficient.
(ii) Another problem with artificial hip joint is that, with the passage of time, the acetabular socket A gradually sinks into the acetabular bone H (what is called "migration") due to repeated hip joint movement and the effect of powder generated during such movement. This hinders the normal function of the hip joint or causes stress concentration at the hip joint, inducing secondary disease. One of the causes of this problem is the fact that the opening edge a1 has no structural relation to the cortex bone H11.